1. Field of the Invention
The present invention relates to a sensor control apparatus that calculates a concentration matching value representing a concentration of a particular gas contained in a detection target gas.
2. Description of the Related Art
In the related art, a sensor control apparatus is known that is installed in an exhaust pipe of an internal combustion engine, such as a vehicle engine, and which detects the concentration of a particular gas composition within gas flowing through an exhaust pipe. The sensor control apparatus includes, for example, a gas sensor, a heater and a control unit. The gas sensor includes a cell having a solid electrolyte body and a pair of electrodes, and outputs an electric signal corresponding to a concentration of a particular gas to the control unit. The solid electrolyte body is generally made of a zirconia-based material, particularly, partially stabilized zirconia from the viewpoint of strength and ion conductivity. In the gas sensor using such a solid electrolyte body, if the temperature of the gas sensor does not reach a predetermined temperature (e.g., 600° C. to 700° C.), the gas detection accuracy based on an electric signal corresponding to a particular gas is degraded. Therefore, when an activation determination temperature used as a reference for determining whether or not the gas sensor is activated is previously set, and a particular gas is detected, the gas sensor is heated by a heater to a temperature equal to or higher than the activation determination temperature. The control unit controls the supply of electric current for heating the heater, and calculates a concentration matching value of a particular gas based on the electric signal output from the gas sensor. In addition, also widely known are a gas sensor that includes an oxygen pump cell and an oxygen concentration detection cell using a solid electrolyte body. The oxygen concentration detection cell autonomously generates an oxygen reference against which the oxygen concentration of an exhaust gas is detected in order to control the supply of electric current to the oxygen pump cell. Also, a control unit for driving the gas sensor is also known (refer to Patent Document 1).
However, in the related art, an automatic stop control is known in which fuel consumption is improved and exhaust gas is reduced by automatically stopping the engine during a temporary stop such as a standby traffic signal. In the automatic stop control, if a condition permitting an automatic stop of the internal combustion engine is satisfied, the internal combustion engine automatically stops. If a condition permitting restart of the internal combustion engine is satisfied, the engine automatically restarts. If an electric current necessary to maintain the temperature of the gas sensor at the activation determination temperature is continuously supplied to the gas sensor during the automatic stop of the internal combustion engine, the power consumption during the automatic stop (hereinafter, also referred to as an “idling stop”) of the internal combustion engine increases, and the load on the battery increases. On the other hand, for example, a control apparatus for an internal combustion engine has been proposed, in which a temperature lower than the activation determination temperature of the gas sensor is set to the target temperature of the heater during idling stop to reduce power consumption of the sensor control apparatus during the idling stop (refer to Patent Document 2).
[Patent Document 1] JP-A-2006-275628
[Patent Document 2] JP-A-2003-148206
3. Problems to be Solved by the Invention
The present inventors found that not just any heater target temperature lower than the activation determination temperature of the gas sensor may be set during the idling stop. That is, the present inventors found that the gas sensor may degrade when the heater target temperature set during the idling stop is not appropriately set. In this regard, the present inventors further discovered that the following problems occur in the type of gas sensor that includes an oxygen pump cell and an oxygen concentration detection cell using a solid electrolyte body, and where the oxygen concentration detection cell autonomously generates an oxygen reference (i.e., where oxygen is pumped from the detection electrode to the reference electrode by supplying electric current to the oxygen concentration detection cell). Specifically, the present inventors discovered that if the solid electrolyte body of the gas sensor continues to be heated to a predetermined temperature (range) lower than the activation determination temperature for a predetermined time period or longer while electric current is supplied to the oxygen concentration detection cell, the solid electrolyte body is prone to blackening. Blackening is a phenomenon where metal is generated on the surface of the solid electrolyte body in the negative electrode side when the metal oxide contained in the solid electrolyte body is chemically reduced (e.g., ZrO2→Zr+O2). In the gas sensor where blackening has occurred, since the ion conductivity of the solid electrolyte body decreases depending on the degree of blackening, it is difficult to detect a particular gas with high accuracy even when the gas sensor is heated to the activation determination temperature. In addition, such problems also occur in a one-cell type gas sensor that includes only an oxygen concentration detection cell, and which measures oxygen concentration depending on the electromotive force generated between a pair of electrodes of the oxygen concentration detection cell while the oxygen concentration detection cell itself autonomously generates the oxygen reference (i.e., where oxygen is pumped toward the reference electrode from the detection electrode by supplying electric current to the oxygen concentration detection cell).